The present invention relates to the reduction of dinitrogen oxide which, for instance, is formed during the catalytic combustion of ammonia and oxygen to nitrogen oxides, which thereupon are cooled in a heat recovery unit and then absorbed in water and/or diluted nitric acid.
Nitric acid is manufactured by catalytic combustion of ammonia with oxygen and subsequent absorption of thereby formed nitrogen oxides in water and/or diluted nitric acid. In modern pressure absorption units one has obtained a substantial reduction of the nitrogen oxide (NO.sub.x) emission in the effluent gases. The effluent gas from the most efficient absorption units contains only about 200 ppm NO.sub.x. It is also known to apply catalytic decomposition of nitrogen oxides to nitrogen and water by reacting NO.sub.x with ammonia over a catalyst in order to meet the environmental emission requirements.
During the catalytic combustion of ammonia there is mainly formed NO and NO.sub.2, but 1-2% of the ammonia is converted to dinitrogen oxide (N.sub.2 O). Investigations have shown that N.sub.2 O does not react or become absorbed in the subsequent process. This implies that all the N.sub.2 O formed during the combustion leaves the plant with the effluent gas. Lately it has begun to be determined whether or not N.sub.2 O, in spite of the fact that it is not very reactive, or possibly just because of its long life time in the atmosphere, can represent detrimental environmental effects. Theoretical model tests indicate that N.sub.2 O can contribute to the destruction of the ozone layer in the atmosphere.
N.sub.2 O, also called laughing gas, is supplied in anesthetic gas in hospitals, and for such small volumes of gases having a high concentration of N.sub.2 O, it is known to decompose N.sub.2 O catalytically. It is, for instance from Japanese patent application No. 55031463, known to perform such decomposition at 150.degree.-550.degree. C. over a platinum catalyst.
It has been further reported in some theoretical studies in the literature about the decomposition of N.sub.2 O. Decomposition of N.sub.2 O is, for instance, used as a model reaction for studying catalysts for selective oxidation. These studies are, however, related to small volumes of gas mixtures having high N.sub.2 O concentration and not to technical units or nitric acid production and the operating conditions which then must be applied.
W. M. Graven, Journal of American Chemical Soc. 81, 6190 (1959) has reported kinetic studies of homogeneous decomposition of N.sub.2 O at 800.degree.-1000.degree. C. In this report it is claimed that the reaction is of first order with regard to N.sub.2 O and that NO has a certain accelerating effect and O.sub.2 a retarding effect. Both decomposition to N.sub.2 +O.sub.2 and to NO.sub.2, was further observed but the decomposition to N.sub.2 +O.sub.2 was dominant. The first order rate constant is stated to be:
k= 2.1.10.sup.9 exp (-220 000/RT) sec.sup.-1
R is in Joule/mol .degree.K. and
T is in degrees .degree.K.
These measurements are, however, carried out in a reaction vessel of 5.6 ml on a N.sub.2 O gas diluted with helium. Small amounts of NO and O.sub.2 were added for studying their influence on the N.sub.2 O decomposition. At increasing NO concentrations it is reported that the reaction N.sub.2 O+NO=NO.sub.2 +N.sub.2 rapidly became dominant.
There is accordingly, no clear guidance to be found in the literature about how one in a technically and economically acceptable way can selectively remove or decompose N.sub.2 O from those gas mixtures being present in the nitric acid plant and under those operating conditions which there exist.